Vulcanization system for acrylate elastomers (trithiocyanuric acid and dithiocarbamic acid derivatives)

ABSTRACT

A vulcanizable acrylic elastomer composition having chlorine or epoxy groups contains as a vulcanization system trithiocyanuric acid and a dithiocarbamic acid derivative. 2,2&#39;&#39;-dithiobis(benzothiazole) is included when maximum scorch protection is desired.

United States Patent Nicholas P. Ermidis West New York, NJ.

Oct. 29, 1969 Nov. 23, 1971 American Cyanamid Company Stamford, Conn.

Inventor Appl. No. Filed Patented Assignee VULCANIZATION SYSTEM FOR ACRYLATE ELASTOMERS (TRITHIOCYANURIC ACID AND DITHIOCARBAMIC ACID DERIVATIVES) 9 Claims, 1 Drawing Fig.

US. Cl 260/795 P, 260/2 EC, 260/861, 260/863, 260/791, 260/793 Int. Cl C08! 27/06 Field of Search 260/795.

References Cited UNITED STATES PATENTS 1/1968 Westlinning 260/415 3/1969 Scherf 260/795 6/1969 De Acelis.. 260/2943 6/1969 Gobran 260/8072 Primary Examiner-Joseph L. Schofer Assistant Examiner-C. A. Henderson, Jr. Auomey-Philip Mintz VULCANIZATION SYSTEM FOR ACRYLATE ELASTOMERS (TRITHIOCYANURIC ACID AND DITHIOCARBAMIC ACID DERIVATIVES) This invention relates generally to acrylic elastomers and, more particularly, to a vulcanizable composition containing an acrylic elastomer having active-halogen or epoxy groups.

Acrylic elastomers have many properties including good heat stability which make them desirable for making various rubberlike products such as, for example, gaskets, hose, conveyor belts, valve seats, packings, oil seals and the like. However, acrylic elastomers are difficult to vulcanize even when they contain vulcanization sites such as active-halogen or epoxy groups. it has been proposed to use an amine, an ammonium salt or an organic peroxide for vulcanizing acrylic elastomers of this type but such vulcanizing systems have had some disadvantage such as slow cure rate, poor curing, or poor aging properties. A soap and sulfur mixture has also been proposed and while it is very suitable for many purposes, it cannot be used to advantage where a fast cure rate with maximum state of cure is required.

It is an object of this invention to provide a novel vulcanizable acrylic elastomer based composition having an improved cure rate. Another object of the invention is to provide a vulcanizable composition containing an acrylic elastomer having active-halogen or epoxy groups and having an improved state of cure. Still another object of the invention is to provide a vulcanizable composition containing an acrylic elastomer having active-halogen or epoxy groups which produces a vulcanizate having improved aging properties. A still further ob ject of the invention is to provide a vulcanizable composition containing an acrylic elastomer combining improved processing safety with improved cure rate, final state of cure and improved aging of the vulcanizate. Another object of the invention is to provide an improved process for vulcanizing a composition containing an acrylic elastomer.

Other objects will become apparent from the following description with reference to the accompanying drawing wherein FIG. 1 compares graphically the thermal stability of embodiments of this invention and of a prior art product as determined by a continuous stress-relaxation technique;

FIG. 2 is a similar graph based on results with intermittent stress-relaxation; and

FIG. 3 compares graphically cure rate data obtained with a Monsanto Oscillating Disc Rheometer.

The foregoing objects and others are accomplished in accordance with this invention, generally speaking, by providing a vulcanizable composition containing an acrylic elastomer and a combination of a dithiocarbamic acid derivative and trithiocyanuric acid in an amount sufficient to vulcanize the composition. The invention further contemplates combining trithiocyanuric acid and a dithiocarbamic acid derivative with 2,2'-dithio-bis( benzothiazole) to provide a vulcanizing system with improved processing safety.

It has been found that although neither trithiocyanuric acid alone nor a dithiocarbamic acid derivative alone is effective for vulcanizing an acrylic elastomer having active-halogen or epoxy vulcanization sites, a combination of the two provides a composition which will cure at a more rapid rate than prior art compositions and will also have a high-state of cure. Moreover, the resulting vulcanizate has improved aging properties.

The invention contemplates broadly all vulcanizable compositions containing any acrylic elastomer having either active-halogen or epoxy groups including those elastomers disclosed, for example, in US. Pat. Nos. 3,201,373 and 3,312,677. More specifically, the invention contemplates acrylic elastomers prepared by polymerizing alkyl acrylates and alkoxyalkyl acrylates, for example, ethyl acrylate, with an epoxy-containing comonomer such as glycidyl methacrylate or glycidyl acrylate and various chlorine or bromine containing copolymers such as, for example, a 95/5 copolymer of ethyl acrylate and chloroethyl vinyl ether. The elastomer may contain both halogen and epoxy vulcanizing sites.

Any suitable dithiocarbamic acid derivative can be used in combination with trithiocyanuric acid in preparing the novel composition provided by this invention. Any suitable thiuram sulfide having the following formula wherein R and R are alkyl, aralkyl, cycloalkyl or heterocyclic when R and R are joined together; and n is an integer of from one to six may be used. Preferably, R and R each have from one to 12 carbon atoms.

Any suitable metal dithiocarbamates having the following formula may be used wherein R and R are alkyl, aralkyl, cycloalkyl or heterocyclic when R and R are joined to form a ring; x is an integer of from one to three and M is a metal such as, for example, zinc, copper, cadmium, lead, bismuth, iron, cobalt, manganese, tellurium or selenium. Preferably, R and R each contain from one to 12 carbon atoms.

Typical examples of thiuram sulfides of the above formula are: tetramethyl thiuram monosulfide; tetramethyl thiuram disulfide; tetraethyl thiuram monosulfide; tetraethyl thiuram disulfide; tetrabutyl thiuram monosulfide; tetrabutyl thiuram disulfide; dipentamethylene thiuram tetrasulfide; bis(morpholinothiocarbonyl)disulfide; cyclohexamethylene thiuram disulfide; tetradodecyl thiuram monosulfide and the like.

Typical examples of suitable metal dithiocarbamates include zinc dimethyldithiocarbamate; zinc diethyldithiocarbamate; zinc dibenzyldithiocarbamate; zinc pentamethylene dithiocarbamate; copper dimethyldithiocarbamate; cadmium diethyldithiocarbamate; lead dimethyldithiocarbamate; bismuth dimethyldithiocarbamate; iron dibenzyldithiocarbamate; tellurium diethyldithiocarbamate; selenium dimethyldithiocarbamate; zinc dicyclohexyldithiocarbamate; zinc didodecyldithiocarbamate and the like.

Best results have been obtained so far with compositions containing tetramethyl thiuram monosulfide so this is the preferred thiuram sulfide. Lead dimethyldithiocarbamate is one of the preferred metal dithiocarbamates because it provides a fast rate of cure and a high-state of cure. Copper dimethyldithiocarbamate is also one of the preferred metal dithiocarbamates because of the improved scorch protection in the vulcanizable composition. When using lead dimethyldithiocarbamate, it is advisable to include 2,2'-dithiobis(benzothiazole) in vulcanizable compositions requiring maximum scorch protection.

The amount of trithiocyanuric acid used can be any amount which will provide a vulcanizable composition. Usually, the amount will be within the range of from about twenty-five hundredths part to about five parts by weight thereof per parts of elastomer in the composition. With most compositions containing an acrylic elastomer, best results are obtained with from about twenty-five hundredths part to about three parts trithiocyanuric acid per 100 parts elastomer so an amount within this range is preferred.

The amount of dithiocarbamic acid derivative likewise can be any amount which will provide a vulcanizable composition but will usually be from about twenty-five hundredths part to about five parts by weight per l00 parts elastomer in the composition. It is preferred to use from about five-tenths to about three parts of the dithiocarbamate derivative per 100 parts elastomer. When 2,2'-dithio-bis(benzothiazole) is combined with trithiocyanuric acid and a dithiocarbamate derivative in the vulcanizable composition, it is preferably used in an amount of from about one-tenth to about three parts by weight per X parts of elastomer.

The vulcanizable composition provided by this invention is prepared by conventional compounding techniques. The various ingredients of the composition may be mixed together on a two-roll mill or in a Banbury mixer. The vulcanizable composition, in addition to the accelerator system provided by the invention, may include the conventional carbon blacks, stearic acid, fillers, antioxidants and similar materials.

In order to illustrate embodiments of the. invention, a masterbatch is prepared by mixing in a Banbury mixer about 100 parts by weight of an acrylic elastomer prepared from 95 percent by weight ethylacrylate and 5 percent vinyl chloroacetate, about one part by weight of stearic acid, about 60 parts by weight of carbon black and about two parts by weight of phenyl-finaphthylamine until a substantially uniform mixture is obtained. The masterbatch is divided into portions and used in the following examples.

EXAMPLE I About 163 parts by weight of the masterbatch prepared as described above are milled on a two-roll rubber mill with about one part copper dimethyldithiocarbamate and about one part trithiocyanuric acid about minutes or until a substantially uniform mixture is obtained.

EXAMPLE [I Example I is repeated except about one part by weight lead dimethyldithiocarbamate is substituted for the copper dimethyldithiocarbamate.

EXAMPLE Ill Example 1 is repeated except about three-tenths part by weight sulfur and about three and five-tenths parts sodium oleate are substituted for the copper dimethyldithiocarbamate and trithiocyanuric acid.

Physical tests are made on the products of examples I, 11 and ill after compression molding and a 5 minute cure at 165 C. with the following results:

Comparison of the 100 percent modulus values indicates that the vulcanization system of this invention reaches a higher state of cure in the same'time over the combination of example [I]. These data also indicate less change in physical properties upon aging of the vulcanizates of examples I and ll.

The thermal stability of cured elastomers prepared from the compositions of examples I, ll and Ill is determined on a six channel, autographic stress-relaxometer. This instrument has load sensing elements, a means for extending and maintaining a test sample at a constant elongation and a circulating oven. Theoven is maintained at a temperature of about 176.6:025" C. The samples are extended lOpercent 10.5 percent.

A sample of cured elastomer from each of examples I, II and III is extended 10 percent on the above instrument and held in this extended condition. Decrease in modulus is continuously recorded. Data relating to the relative thermal stability is obtained from plots otflt)/fl0) as the ordinate versus log time as the abscissa, where flt) and f(0) are the forces at time t and r=0, respectively, required to maintain the sample at the 10 percent extension.

The data are shown in FIG. 1 as T, which represents the time required for the sample to degrade to a value equal to 36.8 percent of the initial stresses. The curves in FIG. 1 show that the cured elastomer of example Ill degrades to 36.8 percent of the initial stresses after 1,300 minutes while those of examples I and I1 require more than 6,000 minutes to degrade to the same extent. These results indicate thermal degradation due to chain scission is greater in the sample of example Ill than either of the other two samples.

Samples of cured elastomer from the compositions of examples i, H and Ill are also compared on the apparatus described above by periodically extending them 10 percent and returning them to rest. Modulus values are determined periodically and plotted in FIG. 2 in the same way as described with respect to FIG. 1. These data indicate the combined effect of cross-linking and chain scission of the samples.

As shown in FIG. 2, the cured elastomer prepared from the composition of example III loses modulus rapidly after repeated stressing and becomes soft and sticky. Note the rapid degradation after 2,000 minutes. With the cured product prepared from the compositions of examples I and II, some cross-linking is apparent with increase in modulus and brittleness but these products are more resistant to degradation than that of example 111. The cured elastomers prepared from the compositions of examples I and ii are still flexible at the end of the test whereas acrylic elastomers cured with sulfur and sodium oleate are brittle and snap.

EXAMPLE IV Example I is repeated except about one part by weight lead dimethyldithiocarbamate alone is used instead of copper dimethyldithiocarbamate and trithiocyanuric acid.

EXAMPLE V Example I is repeated except that about one part by weight trithiocyanuric acid is used alone.

The results obtained in a Mooney Scorch test at 330 F. on the compositions of examples 1V and V at I65 C. for 5 minutes are as follows:

IV V

t,, minutes 8.5 27.5

( minutes EXAMPLE v1 Example ll is repeated except that one and thirty-five hundredths parts by weight lead dimethyldithiocarbamate, one part trithiocyanuric acid and thirty-five hundredths part 2,2- dithio-bis(benzothiazole) are used.

EXAMPLE VII Example ll is repeated except one and seven-tenths parts by weight lead dimethyldithiocarbamate, one part trithiocyanuric acid and seven-tenths part by weight 2,2 '-dithiobis(benzothiazole) are used.

A Mooney Scorch test is made at 250 F. on elastomers prepared by compression molding and curing a sample of the composition of examples ll, VI and V]! for 8 minutes at 165 C. The results of these tests and the physical properties are as follows:

Compression set: Method B, ASTM D-395 70 hrs/150 C.

1: set 31.2 28.8 25.2

Comparison of the Mooney Scorch results demonstrates that improvement is obtained when 2,2'-dithiobis(benzothiazole) is used in combination with the accelerator combination of this invention. Comparison of the physical test data on examples V1 and V1] with those on example II indicates that the 2,2-dithio-bis(benzothiazole) does not adversely affect the physical properties of the cured elastomer.

EXAMPLE VIII Example l is repeated except about one part zinc diethyl dithiocarbamate is substituted for copper dimethyldithiocarbamate. The Mooney Scorch and physical properties are:

Mooney Scorch t minutes I minutes Oscillating Disc Rheometer lll Torque alter 5' (inch pounds) l0 Properties Cure, minutes T F. Tensile. p.s.i. Elongation, b Modulus, lOOii. p.s.i. Hardness. Shore A EXAMPLE IX Example I is repeated except about one part bismuth dimethyldithiocarbamate is substituted for copper dimethyldithiocarbamate. The physical properties are:

Oscillating Disc Rheometer 1 F. 330 Torque alter 5 83 (inch pounds) I0 105 20' 116 Properties Cure, minutes/l" F. l0l330 Tensile, p.s.i. 1,695 Elongation. 230 Modulus, l00%, p.s.i. Hardness, Shore A 70 EXAMPLE X Example I is repeated except about one part zinc dibutyldithiocarbamate is substituted for copper dimethyldithiocarbamate. The physical properties are:

Oscillating Disc Rheorneter T F. 330 Torque after 5' 42 (inch pounds) 10 116 20' I35 Properties Cure, minutes/T F. l0'/330 Tensile, psi. 1,500 Elongation, Z: I50

Modulus, p.s.i. Hardness, Shore A EXAMPLE IX Example I is repeated except about one part zinc dibenzyldithiocarbamate is substituted for copper dimethyldithiocarbamate. The physical properties are:

Oscillating Disc Rheometer T F. 330 Torque after 5' 19 (inch pounds) 10' 47 20 9| Properties Cure, minutes/P F. l0l330 Tensile, p.s.i. I ,490 Elongation, 36 I90 Modulus, l00%, psi. 880 Hardness, Shore A 79 EXAMPLE XI] Example I is repeated except about one part zinc pentamethylenedithiocarbamate is substituted for copper dimethyldithiocarbamate. The Mooney Scorch and physical properties are:

Mooney Scorch t,, minutes 0.6 t minutes 0.6 Properties Cure, minutes/T F. 107330 8 Tensile. p.l.i. L795 (inch pound!) I 94 Elon ltion, lSO l l00 Modulus, l00lw, p.s i. l,4l0 20' I04 Hardness, Shor A 77 EXAMPLE XVII EXAMPLE Xlll Example ll is repeated except one and five-tenths parts lead Example l is repeated except about one part cadmium dimethyldimiocarbamam an sect diethylditluocarbamate is substituted for copper dimethyllo The composition prepared in example In and the composi 'q' The Mommy Scorch and physcal Pmpcmes tion of example XVII are cured for 8 minutes at 330 F. The following test results are obtained:

" xvn lll Z: :inum 8 Physical properties IA, minutes 0.4 Properties Tensile, p.s.i. l.9l5 1,300 Elongation, at I 380 Cure, minutes/l F. 257330 l: 7,: Z: Tenlile, p.r.i. 1,575 Elongation. es 200 Modulul, 100%, p.|.i. l,l25 A 73 25 Comparison of these results indicates that a product prepared in accordance with this invention is fully cured after 8 minutes at 330 F. but the product of example lll using the EXAMPLE XIV gigs-2n soap and sulfur system has reached only a low-state Example l is repeated except about one part tellurium diethyldithiocarbamate is substituted for copper dimethyl- EXAMPLE dithlocarbamate- The Physlcal Pmpemes A vulcanizable composition is prepared by mixing in a Banbury mixer about 100 parts of an elastorner having the composition of 98 percent ethylacrylate and 2 percent glycidyl P acrylate, about one part stearic acid, about two parts phenyl- B-naphthylene, about 60 parts carbon black. Aboutone part Cure minutes/T F. 87330 trithiocyanuric acid and about one and five-tenths parts lead dimethyldithiocarbamate are then mixed on a two-roll rubber Modal. {00%, PM 800 mill with the mixture obtained in the Banbury mixer. Hardness, shim A 82 The following physical test data is obtained on the product after curing for 15 minutes at 330 F:

EXAMPLE XV Example I is repeated except about one part iron dimethyl- Pwpmi dithiocarbamate is substituted for copper dimethyldithiocarbamate. The Mooney Scorch and physical properties are: P-=- 1-525 Elongation, k 275 Modulus, l00k, p.s.i. 705 Shore A hardness 70 Mooney Scorch Oscillating Disc Rheometer 330 F.

T F. 330 Torque after 5' 49 t.. minutes 0.9 I0 52 IA. minutes 0.4 55 I5 53 Properties Cure, minutes/l F. 8'1330 The results obtained with the Theometer indicate a relative- S -F s; ly rapid cure rate when the combined accelerator system of PM "3'0 this invention is used with an epoxy-containing acrylate Hardness, Shore A 75 elastorner. It should be noted that the cure rate has leveled off after about 10 minutes at 330 F.

A masterbatch is prepared by mixing a Banbury mixer about 100 parts of an elastorner having the composition of 95 per- EXAMPLE XVI cent ethylacrylate-butylacrylate (82-18 percent) and 5 pert vinyl chloroacetate, about two parts stearic acid about Example I IS repeated except about one and five-tenths can parts tetramethyl thiuram disulfide is substituted for copper g: :L hfspar y -para-phenylene diamme and dimethyldithiocarbamate. The physical properties are: a u pa car n ac EXAMPLE XlX A portion of the immediately foregoing masterbatch is omlminl Dix comm, mixed on a two-roll rubber mill with about two parts lead dimethyldithiocarbamate, about one part trithiocyanuric acid F 330 and about one part 2,2'-dithio-bis(benzothiazole) per 100 parts of elastorner in the masterbatch.

Torque after 5' EXAMPLE xx Example XVIII is repeated except that about three and fivetenths parts sodium oleate and about three-tenths part sulfur are substituted for the lead dirnethyldithiocarbamate, tr'ithiocyanuric acid and 2,2'-dlthlo-bls(benzothlazole).

The compositions of examples XIX and XX are cured for about 8 minutes at 165 C. and tested with the following results:

Comparison of the foregoing results again illustrates that the vulcanizable composition provided by this invention reaches a higher state of cure than that of the prior art soap and sulfur system. The curves of torque versus time at 330 F. with an oscillating disc Rheometer have been plotted in FIG. 3. The curves indicate that the composition of example XIX obtains its maximum state of cure very rapidly whereas that of example XX is very slow. After 16 hours at 150 C., the composition of example XIX attains a significantly higher tensile strength than that of example XX.

A masterbatch is prepared by mixing in a Banbury mixer about 200 parts by weight of a 95 percent ethylacrylate and percent vinyl chloroacetate elastomer, about two parts stearic acid and about 120 parts carbon black.

EXAMPLE XXI About one part by weight trithiocyanuric acid, about one part 2,2'-dithio-bis( benzothiazole), about two parts phenyl-B- naphthylamine and about two parts lead dimethyldithiocarbamate are mixed on a two-roll rubber mill with about I61 parts of the immediately above masterbatch.

EXAMPLE XXII XX] XXII t,, minutes (250' F.) 18 26.5 t,, minutes (330 F.) 1.6 2.3 0.6 1.3

t minutes (330' F.)

After curing for 8 minutes at about 330-340 F. and postcuring 16 hours at 150 C. the compositions have the following physical properties:

XXI XXII Tensile, p.s.i. 1,790 1,665 Elongation, b 185 210 Modulus, 100% 1,110 845 Shore A hardness 78 79 It is apparent from the above data that the composition of the invention as represented by example XXI cures faster, provides adequate processing safety and reaches a higher state of cure than that of the prior art represented by example XXII.

EXAMPLE XXIII About parts by weight of the elastomer used in example I, about one part stearic acid and about 60 parts carbon black are mixed together in a Banbury mixer. The resulting mixture is then mixed on a two-roll rubber mill with about two parts phenyl-flmaphthylamine, about one part copper bis(morpholinothiocarbonyl)disulfide and about one part trithiocyanuric acid.

The Mooney Scorch at 280 F. (t,,, minutes) is 13.5 and the cure rate as determined with an Oscillating Disc Rheometer at 3 30 F. is five.

EXAMPLE XXIV About 100 parts of an elastomer of the composition of 95 percent ethylacrylate and 5 percent vinyl chloroacetate, about one part stearic acid and about 60 parts carbon black are mixed together in a Banbury mixer. The resulting mixture is then mixed with about two parts phenyl-fi-naphthylamine, about one part trithiocyanuric acid and about 1.5 parts tetramethyl thiuram monosulfide on a two-roll rubber mill.

The following results are obtained in tests on the product:

Oscillating Disc Rheometer at 176.6 C.

Cure rate 7.0 Torque at 90% ofcure, in. lbs. Time to 90% of cure, minutes Physical properties: Cured l0 min/165 C.

Postcured 16 hrs/ C.

Tensile, p.s.i. Elongation, ii: Modulus, 100 i: Shore A hardness Example XXIV is repeated except about one part bis(morpholinothiocarbonyl)disulfide is substituted for tetramethyl thiuram monosulfrde.

The following physical test data is obtained on the product:

Oscillating Disc Rheometer at 176.6 C.

Cure rate 5.8

Torque at 90% of cure, in. lbs.

Time to 90% of cure, minutes Physical properties: Cured l0 min/165 C.

Postcured l6 hrs/150 C.

Tensile, psi. Elongation, Modulus, 100% Shore A hardness wherein R and R are alkyl, aralkyl, cycloalkyl or heterocyclic when R and R' are joined together to form a ring and n is an integer of from I to 6.

MAR

4. The composition of claim 1 wherein the dithiocarbamic acid derivative is zinc, copper, cadmium, lead, bismuth, iron, cobalt, manganese, tellurium or selenium dithiocarbamate.

5. The composition of claim 1 containing 2,2'-dithiobis(benzothiazole).

6. The composition of claim 5 containing from about 0.1 to about three parts by weight per parts of elastomer of 2,2 dithio-bis-(benzothiazole).

7. The composition of claim 4 wherein the metal dithiocarbamate is lead dimethyldithiocarbamate.

8. The composition of claim 4 wherein the metal dithiocarbamate is copper dimethyldithiocarbamate.

9. The composition of claim 1 wherein the dithiocarbamic acid derivative is tetramethylthiuram monosulfide.

i I! i i l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 15h7 Dated November 23, 1971 ImMntOI-(S) Nicholas P. Ermidis It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1;, line 7, "10 percent f 0.5 percent" should read --l0 percent 1- 0.05 pereent--.

Column 5, line 31, after "ASTM" "0D" should read #D--.

Column 10, line LLS, insert the heading --EX.AMPLE XXV-.

Signed and sealed this 9th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCH.ER, JR. ROBERT GOT'ISCHALK Attesting Officer Co missioner of Patents oRM P0-1050 (10-69) uscoMM-Dc Burro-P69 & \lS. GDVERNMENY PRINTING OFFICE "If O3il-SM 

2. The composition of claim 1 wherein from about 0.25 to about five parts by weight each of trithiocyanuric acid and of a thiuram sulfide or a metal dithiocarbamate per 100 parts elastomer are included in the composition.
 3. The composition of claim 1 wherein the dithiocarbamic acid derivative is thiuram sulfide having the formula
 4. The composition of claim 1 wherein the dithiocarbamic acid derivative is zinc, copper, cadmium, lead, bismuth, iron, cobalt, manganese, tellurium or selenium dithiocarbamate.
 5. The composition of claim 1 containing 2,2''-dithio-bis(benzothiazole).
 6. The composition of claim 5 containing from about 0.1 to about three parts by weight per 100 parts of elastomer of 2,2''-dithio-bis-(benzothiazole).
 7. The composition of claim 4 wherein the metal dithiocarbamate is lead dimethyldithiocarbamate.
 8. The composition of claim 4 wherein the metal dithiocarbamate is copper dimethyldithiocarbamate.
 9. The composition of claim 1 wherein the dithiocarbamic acid derivative is tetramethylthiuram monosulfide. 